MDCT Technology
Kalpana M. Kanal, Ph.D., DABR
Assistant Professor
Department of Radiology
University of Washington
Seattle, Washington
ACMP Annual Meeting 2008 - Seattle, WA
Educational Objectives
Historical Development of CT
Why has the frequency of CT increased?
Multi-Row Detector CT and Pitch
Advantages of Multi-Row Detector CT
Automatic Exposure Control in CT
Cardiac CT
Future of CT
Kanal
2
Historical Development of CT
64 slice
scanners
Dual Source
CT
2004- 2005
Slow scan time
Fast scan time
Dr. Mahesh, John Hopkins, MD, AAPM Handout.
Kanal
3
Historical Development of CT
Seventh Generation CT (late 1990’s – 2000)
Multi-Row Detector Systems
Radiographics 2000: 2020-17871787-1806
Kalendar WA, Computed Tomography,
Second Edition, 2005, pg 58.
Kanal
4
Historical Development of CT
PET/CT Scanners (2001)
Kanal
Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 74.
5
Historical Development of CT
Dual-Source CT (2005)
A scanner using two
X-ray sources and two
detectors at the same
time
Enables scanning a
heart at any heart rate
and at the lowest
radiation dose
possible
Kanal
www.medical.siemens.com
6
Fast Scanning, 0.3 sec
Kanal
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Why has the frequency of CT exams
increased?
New
technology
Faster scans
New exams
done with CT
More
screening
exams
UNSCEAR 2000
More like 60%
Kanal
ICRP Publication 87 (2001)
8
Single-Row Detector versus Multi-Row
Detector CT
Kanal
Radiographics 2000: 2020-17871787-1806
9
Single-row Detector vs. Multi-row Detector
CT
Kanal
Seeram, Computed Tomography, 2nd ed., 2001, pg 258.
10
Multiple Detector Arrays
Kanal
Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 341.
11
Slice Width Selection
GE 64 MDCT
(40 mm
detector)
64 x 0.625 mm
32 x 1.25 mm
16 x 2.5 mm
8 x 5 mm
http://www.impactscan.org/slides/impactcourse/helical_and_multi-slice_principles/img17.html
Kanal
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32/40/64 Slice CT Systems
http://www.impactscan.org/slides/ukrc2005/img11.html
Kanal
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Pitch in Multi-slice CT
Two pitch definitions seen in MSCT
Collimator pitch = table travel_____
x-ray beam width
Detector pitch = table travel
detector width
Collimator pitch = Detector pitch
N
Kanal
c.f. http://www.impactscan.org/slides
14
Multi-Row Detector CT - Advantages
Same acquisition in shorter times (fewer motion artifacts)
Kanal
Seeram, Computed Tomography, 2nd ed., 2001, pg 263.
15
Multi-Row Detector CT - Advantages
Reconstruction in different slice widths
Kanal
http://www.impactscan.org/
16
Multi-Row Detector CT - Advantages
Efficient use of x-ray beam with increasing slices
Reduction of radiation dose
Kanal
Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 168.
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Multi-Row Detector CT - Advantages
Thinner slices for better z-axis resolution
Isotropic Imaging
Improved Spatial
Resolution
All sides of the voxel
have equal dimensions
Kanal
Seeram, Computed Tomography, 2nd ed., 2001, pg 264.
18
Multi-Row Detector CT - Advantages
Better Multiplanar Reconstruction & 3D Display
Kanal
Radiographics 2002: 2222-12551255-1268
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Regarding Multiple Detector CT
scanners:
14% 1)
A scanner with a 16-detector array employs a total of 16x16
detectors
14% 2)
Detectors used are the non-radioactive xenon-gas detectors
3)
The slice width is determined by width of detectors in slice
thickness dimension
The slice width selected can be smaller than the physical
size of the detector
The spatial resolution is worse than that of a single-detector
CT scanners
29%
43% 4)
0%
Kanal
5)
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Answer
The slice width is determined by width of detectors in
slice thickness dimension
Kanal
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Multi-Row Detector CT - Advantages
Cardiac CT
40 mm detector
(Pitch ~0.25)
12.5 cm in 5 sec
20 mm detector
(Pitch ~0.25)
6.2 cm in 5 sec
Kanal
Kalendar WA, Computed Tomography, 2nd ed., 2005, Video.
22
For a 16-detector row CT scanner, the x-ray beam
width is 20 mm, detector width is 5 mm and the
table travel per rotation is 20 mm. Calculate the
detector pitch and collimator pitch?
15%
62%
15%
8%
0%
Kanal
1.
2.
3.
4.
5.
1, 4
4, 1
5, 20
4, 4
20, 5
23
Answer
4, 1
Pitchd = 20 = 4 = Detector pitch
5
Pitchx = 20 = 1 = Collimator pitch
20
Kanal
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Dose Reduction
Current Modulation or AEC in CT
Modern CT scanners are capable of modulating the mA
(current) during the scan
Rationale: it takes fewer photons (lower mA) to penetrate
thinner tissue, and more x-ray photons (higher mA) to
penetrate thicker projections through the body
Benefits
Consistent image quality
Dose reduction potential
Reduced tube loading
Extended scan runs
Reduction in photon starvation artifact
Kanal
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Automatic Exposure Control (AEC) in CT
Patient size AEC
Patient size AEC: higher mA used for larger patients
Kanal
MHRA Report 05016, February 2005
www.isteve.com
www.yuzbasicafer1.com
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Automatic Exposure Control (AEC)
Z-axis AEC
Z-axis AEC: higher mA used at more attenuating
positions along the z-axis (patient)
Kanal
MHRA Report 05016, February 2005
Radiographics, 2006, 26:503-512
27
Automatic Exposure Control (AEC) in CT
Angular AEC
Angular AEC: the degree of modulation depends on the
asymmetry of the patient
Kanal
MHRA Report 05016, February 2005
Radiology, 2004, 233:649-657
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Dose Reduction
Cardiac ECG dose modulation
It is possible to vary the mA based on R-R interval phase %
This can lead to a dose decrease of up to 50% in low stable
heart rates
70%
mA
80%
interval used
for image
reconstruction
dose not given to patient
550
200
t
Kanal
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Dose Reduction - ECG dose modulation
90
HR 30-74 BPM (One
Segment)
80
Exposure CTDI (mGy)
70
60
50
40
30
20
Segment mode, no ECG
Coverage during
scan
10
0
0
50
100
150
200
250
240o
Heart rate (BPM)
Kanal
Paul Kinahan and Kelly Branch, UWMC
30
Dose Reduction - ECG dose modulation
HR 30-74 BPM
(One Segment),
TR 175 msec
90
Exp o su re C T D I (m G y)
80
70
42%
decrease
in dose
60
50
40
30
Needed coverage
during scan
Segment mode, no ECG
20
Segment mode [65-85%], w/ECG
10
Segment mode [70-80%], w/ECG
0
0
Kanal
50
100
150
Heart rate (BPM)
200
250
240o
Paul Kinahan and Kelly Branch, UWMC
31
Regarding automatic tube current
modulation in CT:
0%
1.
It gives inconsistent image quality
0%
2.
It increases the scan time for the exam
0%
3.
4.
It generates more heat in the x-ray tube
The tube current is automatically adjusted according to
patient size
It causes an increase in photon starvation artifact
100%
0%
Kanal
5.
32
Answer
The tube current is automatically adjusted according to
patient size
Kanal
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Reducing Cardiac Dose – Prospective
Gating
No functional information
Kanal
Mahesh, et al. Radiographics, 2007; 27:149527:1495-1509.
Retrospective Gating
Kanal
Mahesh, et al. Radiographics, 2007; 27:149527:1495-1509.
Prospective vs. Retrospective Gating
University of Washington Study
Effective
Dose (mSv)
Retrospective
Gating
26.4 ±6.2
Prospective
Gating
6.2 ± 2.0
77% lower radiation dose with prospective study
This study concluded that the “64 channel cardiac CT
performed with prospective ECG gating may have similar
subjective image quality and substantially lower patient
radiation dose compared to retrospective ECG gating”
Kanal
Shuman, et al. submitted for publication, 2007
Regarding prospective ECG triggering
for cardiac CT exams
0%
1.
Projection data has to be collected through out the cardiac
cycle
0%
2.
The temporal resolution is about 350 ms
3.
The radiation dose is reduced compared to retrospective
100%
ECG gating
0%
4.
0%
5.
Kanal
Functional information of the heart is available with this
exam
Increase in motion artifacts occurs
37
Answer
The radiation dose is reduced compared to
retrospective ECG gating
Kanal
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Future of CT
Kanal
Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 235.
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Future of CT
Other Developments
Kanal
Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 72.
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TAKE HOME POINTS
MDCT scanners are becoming common and have several
advantages over SDCT
CT utilization has increased significantly
AEC or Automatic Tube Current Modulation and ECG
modulation available on modern scanners which helps
reduce dose
Cardiac CT is becoming popular with MDCT scanners
Always keep ALARA in mind!
Kanal
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THANK YOU
Kanal
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